For a frequency analysis, I need to model the restraints of a box placed on a base plate.
In the real world, the box will be fixed to a base plate with four screws.
In the simulation, I consider only the box, so I set the screw holes as "fixed surfaces", but I would also like to model the contact between the lower face of the box and the base plate to inhibit the movement of the lower surface of the box downwards along the vertical axis. In fact, that face will not be able to break through the base plate, while it will be able to deviate upwards along the vertical axis.
How can I model a no penetration restraint along a single direction of an axis perpendicular to a plane, leaving the movement free along the opposite direction?